Multiple driven axle vehicles, such as four wheel drive vehicles have utilized a variety of means for accomplishing four wheel drive function. Difficulties encountered in designing multiple axle drive systems have given rise to a variety of proposed solutions. A significant problem encountered is providing an effective and economical differential between left and right wheels on the steering axle and between axles during four wheel driving to prevent tire scrubbing which not only causes excessive tire wear but also impairs the driver's ability to control the vehicle. Many vehicles, such as automobiles, utilize relatively expensive and heavy differentials between wheels on the same axle and transfer cases between axles to permit all wheels to rotate at different speeds, thus preventing tire scrubbing and handling problems during cornering.
Due to a variety of reasons relating to such things as handling and economy, four wheel drive vehicles such as pick ups typically require the optionally driven axle to be manually engaged by the driver. Thus, when four wheel drive is not required, the vehicle is operated in two wheel drive mode; the optionally driven wheels ar engaged manually by the driver when the driver considers four wheel drive to be necessary or desirable. Such systems have obvious disadvantages in that they are not automatic and the driver must continually be conscious of the condition of the driving surface to decide whether two wheel drive or four wheel drive is appropriate.
Vehicles such as all terrain vehicles (ATV's) typically are driven on all types of terrain including pavement, grass, dirt, snow, ice, gravel, and even shallow water. Such vehicles frequently are utilized off road where terrain changes occur frequently and rapidly. Four wheel drive capability is particularly valuable for such vehicles to enhance their ability to travel safely over all types of terrain without getting stuck. Four wheel drive systems on ATV's typically have included either a full slip differential or a limited slip differential on the front axle. Either type of front differential allows the vehicle to be steered without causing undue scrubbing of the tires, but does not provide true four wheel drive operation. If all wheels are on a slippery surface except one of the front wheels, three wheels will spin and the fourth will not drive. This problem is not entirely alleviated by even the limited slip differential. In some full slip front differential systems, the front differential can be locked-in during very low speed operation of the vehicle. Typically such systems require the vehicle to be completely stopped to lock in the differential, necessarily resulting in loss of all vehicle momentum which often allows the vehicle to get stuck. Steering is also somewhat difficult, since all four tires must then rotate at the same speed.
Double acting overrunning clutches have long been used in mechanical operations to allow a driven member to rotate faster than a drive member. See, e.g., U.S. Pat. No. 3,194,369, W. Witte, Double Acting Overrunning Clutch, July 13, 1965. Roller clutches have been used in transfer cases of four wheel drive vehicles to engage the front axle when the rear wheels begin to spin. See U.S. Pat. No. 3,295,625, M. Ordorica, Automatic Four Wheel Drive Transfer Case, Jan. 3, 1967. Roller clutches have also been used in hubs for the front wheels of four wheel drive vehicles. See U.S. Pat. No. 3,788,435, Prueter, Automatic Hub for Four Wheel Drive Vehicles and the Like, Jan. 29, 1974.
It is believed that none of these prior art systems have successfully utilized roller clutches for both side-to side and front to rear differential function as well as providing for automatic engagement of the optionally driven wheels. Furthermore, in applying these prior art systems to four wheel drive vehicles a number of problems have been encountered which prevent safe and efficient operation of the under certain driving condically. Specifically such vehicles typically are driven harder than on-road vehicles such as automobiles and pick-up trucks. During four wheel drive operation, the vehicles encounter frequent stop and start situations, hard braking followed by rapid acceleration, and tight cornering on a relatively short wheel base. For reasons which will be explained in greater detail below, such conditions can cause undesirable locking up of the four wheel drive system in certain circumstances.